Chemical mechanical planarization of low dielectric constant materials

ABSTRACT

The present invention relates to apparatus, procedures and compositions for avoiding and reducing damage to low dielectric constant materials and other soft materials, such as Cu and Al, used in fabricating semiconductor devices. Damage reduction can be achieved by decreasing the role of mechanical abrasion in the CMP of these materials and increasing the role of chemical polishing, which can also improve material removal rates. Increasing the role of chemical polishing can be accomplished by creating a polishing slurry, which contains components that interact chemically with the surface to be polished. This slurry may or may not also contain soft abrasive particles, which replace the hard abrasive particles of conventional slurries. Use of soft abrasive particles can reduce the role of mechanical abrasion in the CMP process. Use of this slurry in CMP can reduce surface scratches and device damage.

This application is a divisional of allowed application Ser. No.09/547,187, filed Apr. 11, 2000, now U.S. Pat. No. 6,416,685.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to the chemical mechanical planarizationof surfaces. More particularly, the present invention relates to theplanarization of relatively soft materials, typically low dielectricconstant materials as encountered in the fabrication of integratedcircuits.

2. Description of Related Art

Fabrication of integrated circuits (“ICs”) to improve performance andreduce costs involves complex analysis of materials properties,processing technology and IC design. IC's consist of multiple layers ofconducting, insulating and semiconductor materials, interconnected invarious ways by conducting metallic channels and plugs (“vias”),including various dopants implanted into various materials for producingthe electronic functionality desired of the IC. The near-universal trendin the manufacture of integrated circuits is to increase the density ofcomponents fabricated onto a given area of wafer, increase theperformance and reliability of the ICs, and to manufacture the ICs atlower cost with less waste and fewer defective products generated by themanufacturing process. These goals lead to more stringent geometric anddimensional requirements in the manufacturing process. In particular,etching precise patterns into a layer is facilitated by the layer havinga surface as nearly planar as feasible at the start of the patterningprocess. For the common case of patterning by means of photolithography,a planar surface permits more precise location and dimensioning forfocusing the incident radiation onto the surface to be etched than wouldbe possible with a surface having deviations from planarity. Similarconclusions typically apply for electron beam or other means of etching.That is, deviations from planarity of the surface to be etched reducethe ability of the surface to support precisely positioned and preciselydimensioned patterns. In the following description of the presentinvention we focus on the typical etching, planarization andphotolithography processes as practiced in the manufacture of ICs.However, this is by way of illustration and not limitation, as those ofordinary skill in the art of etching will appreciate that the techniquesof the present invention producing planar surfaces will haveapplicability in increasing the precision of etching by means other thanphotolithography. In addition, the present invention is not limited tothe field of IC manufacture and may find applicability in other areas oftechnology requiring planar surfaces.

Chemical Mechanical Planarization (“CMP”) has been successfullyintegrated into integrated circuit multilayer manufacturing processes toachieve highly planar surfaces as described in text books (for example,“Microchip Fabrication” by Peter Van Zant, 3rd Ed., 1997) and generallyknown in the art. We note that “CMP” is also used in the art to denote“Chemical Mechanical Polishing” as well as “Chemical MechanicalPlanarization”. We use CMP herein synonymously in either sense withoutdistinction.

A typical CMP process is depicted schematically in FIG. 1. During a CMPprocess, the wafer, 1, is typically held inside a rotating carrier andpressed onto a rotating pad, 2, under pressure, 6, while an abrasiveslurry, 5, (typically containing particles of abrasive such as SiO₂,Al₂O₃, and the like) flows between the wafer and the pad. The slurry, 5,will typically contain reagents for chemically etching the wafer, 1,leading to chemical as well as mechanical removal of material. Thus, inthe typical practice of CMP, material removal is effected by acombination of chemical attack and mechanical abrasion.

Typically, the wafer, 1, will be caused to rotate as depicted by 4 inFIG. 1, while the polishing pad will itself rotate (3 in FIG. 1). FIG. 1depicts the polishing pad and wafer rotating in the same direction (forexample, clockwise when viewed from above as in FIG. 1). However, thisis merely for purposes of illustration and counter-rotation of wafer andpolishing pad is also practiced. In addition to the rotation of thewafer depicted by 4 in FIG. 1, the wafer, 1, may be caused to oscillatein the plane of the surface being polished, substantially perpendicularto the direction of the applied force, 6. Such oscillation is depictedas 7 in FIG. 1.

The necessary parameters for polishing SiO₂-based intermetal dielectriclayers occurring in ICs have become well known in the semiconductorindustry. The chemical and mechanical nature of polishing and wear ofthese SiO₂-based dielectric layers (“SiO₂ dielectrics”) have beenreasonably well developed. One problem with the SiO₂ dielectrics,however, is that the dielectric constant is relatively high, beingapproximately 3.9. Thus, to improve the electrical performance of ICs,it would be highly desirable to incorporate a low dielectric constantmaterial into semiconductor structures while still being able to utilizethe CMP systems for polishing the surface of the resulting dielectricmaterial during the semiconductor wafer processing.

As the geometry of the integrated circuits continues to shrink, theintrinsic circuit delays will increase due to greater resistance in themetal interconnects and also due to undesired (“parasitic”) capacitanceeffects arising from the circuit interconnects. Strategies beingdeveloped to reduce the parasitic capacitance effects includeincorporating metals with lower resistivity values, such as copper, andproviding electrical isolation with insulating materials having lowdielectric constants relative to the SiO₂ dielectrics.

As described herein, “low dielectric constant materials” may occur innumerous physical and chemical forms, including organic polymermaterials, porous dielectric materials, whether organic or inorganic,and mixed organic and inorganic materials, whether porous or not(examples include FLARE, SILK, HOSP, NANOGLASS, ELK, and Polyimides).Typically these low dielectric constant materials are polymer dielectricmaterials which include unique chemical and mechanical characteristics,including a relatively high concentration of organic materials. However,low dielectric constant materials may also include relatively highlyporous materials (typically inorganic) or materials exhibiting a mixtureor combination of properties and characteristics (organic, polymeric,porous, inorganic, etc.). The low dielectric constant films cantypically be deposited utilizing a variety of techniques includingchemical vapor deposition (CVD), physical vapor deposition (PVD) andspin coating. The polymer materials generally are mechanically soft andthey readily exhibit plastic deformation and hence they easily can bescratched. In contrast, however, to their mechanical sensitivity,polymers are often chemically inert. The combination of thecharacteristics of the polymer dielectric materials makes use of aconventional aqueous based CMP process difficult.

Theoretically and practically, use of a hard CMP pad results in betterplanarity of the polished wafer while use of a soft CMP pad providesbetter surface qualities and uniformity of the polished films. Thecurrent generation of semiconductor devices (which are typically madecontaining tungsten and oxides) are typically planarized using CMP withan industry standard hard pad, such as the IC® pad, for primaryplanarization and a standard soft pad, such as the Politex® pad, forsecondary buff polishing. However, using the standard IC® pad and theconventional abrasive slurries (SiO₂, Al₂O₃, CeO₂, and the like) topolish low dielectric constant materials, which typically are muchsofter than the conventional materials used in integrated circuits,tends to cause significant scratches on the polished surfaces of lowdielectric films. Attempts to use soft pads like Polytex® pads to polishthese soft materials have shown some success in avoiding severescratches, but do not efficiently achieve good planarity in reasonableprocessing times. (S. P. Murkarka and R. Gutmann, 1994 Annual Report ofthe New York State SCOE, Semiconductor Research Corporation, ResearchTriangle Park, N.C. (1994)).

The abrasive particles typically used in conventional polishing slurrieshave a comparable mechanical strength, or hardness, to that ofconventional device materials, such as oxide and tungsten. However,these particles' mechanical strength is typically much higher than thatof low dielectric constant materials and other soft materials used inmore recent embodiments of ICs, such as copper or aluminum. Duringpolishing, the hard abrasive particles tend to penetrate deeply into thefilms of these soft materials and cause severe scratches. ConventionalCMP slurries are also typically composed of inorganic chemicalcomponents that are reactive to inorganic materials such as oxides andmetals, but do not actively react with the organic polymers thattypically form the low dielectric constant material layers on thesurface being planarized. Therefore, when conventional CMP methods areused in an attempt to planarize low dielectric constant materials,mechanical abrasion dominates the process, resulting in increasedscratching and device damage. These surface scratches in device waferscan cause severe problems for subsequent processing steps, and cansignificantly reduce yields.

Thus, current standard hard and soft pads, and conventional abrasiveparticles and slurries are not suitable for CMP of softer materials suchas low dielectric constant materials. Successful incorporation of theselow dielectric constant materials into viable submicron fabricationtechniques will necessitate the development of CMP processes which arenot currently available utilizing the SiO₂-based CMP processes. Improvedplanarization of soft materials, typically low dielectric constantmaterials, is an important objective of the present invention.

SUMMARY OF THE INVENTION

The present invention relates to apparatus, procedures and compositionsfor avoiding and reducing damage to low dielectric constant materialsand other soft materials, such as Cu and Al, used in fabricatingsemiconductor devices. Damage reduction can be achieved by decreasingthe role of mechanical abrasion in the CMP of these materials andincreasing the role of chemical polishing, which can also improvematerial removal rates. Increasing the role of chemical polishing can beaccomplished by creating a polishing slurry which contains componentsthat interact chemically with the surface to be polished. This slurrymay or may not also contain soft abrasive particles, which replace thehard abrasive particles of conventional slurries. Use of soft abrasiveparticles can reduce the role of mechanical abrasion in the CMP process.Use of this slurry in CMP can reduce surface scratches and devicedamage.

Polishing pads with intermediate hardness are also included within thescope of the present invention. Such pads may be used with or withoutabrasive slurries and, when used with abrasive slurries, may be usedwith conventional slurries or with softer-than-customary abrasiveparticles (or coated abrasive particles). A balance is achieved betweensmooth surface finish and efficiency of polishing (material removalrate) by adjusting the polishing pad hardness to the surface to beplanarized.

A two (or multi) step process is also envisioned within the scope of thepresent invention as a first planarization step (which may beconventional CMP or any of the improvements noted herein), followed by abuffing step. The subsequent buffing step(s) need not remove significantmaterial as this has typically been accomplished by the precedingplanarization step. Thus, the buffing step may be abrasive-free, utilizesoft (or coated) abrasives and typically is gentler than a one-stepprocess as the surface needs only to be buffed without substantialmaterial removal as would be necessary for planarization.

BRIEF DESCRIPTION OF DRAWINGS

The drawings herein are not to scale.

FIG. 1 is a schematic depiction of Chemical Mechanical Planarization(“CMP”) including wafer, polishing pad and pad conditioning implements.

FIG. 2 a schematic cross-sectional view of a typical multi-layerintegrated circuit undergoing planarization.

FIG. 3 results of mathematical model examining combined effect of wt %abrasive, abrasive diameter and polishing pressure on particlepenetration depth δ, cumulative contact area and compressive contactstress σ.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Chemical Mechanical Planarization (CMP) has been successfully integratedinto fabrication processes for integrated circuits containing multiplelayers. Dielectric CMP is performed typically to remove topography fromthe dielectric surface, that is, to planarize the dielectric surface.Such topography typically originates from the deposit of dielectricmaterial on top of underlying metal structures. Metal CMP is typicallyperformed to remove a blanket metal film thus revealing inlaid metalstructures that act as conductive elements in the integrated circuits(IC's). Current IC designs incorporate multiple layers of conducting,insulating and semiconductor layers, typically having patterns thereinformed by means of photolithography. The critical dimension requirementsof current IC's require a wafer surface that is highly planar beforebeginning the photolithography patterning, hence the need for a veryprecise planarization process. Currently, conventional CMP is performedon polishing platforms with mechanical designs that are based on siliconwafer polishing tools. During CMP, an oscillating and rotating wafer, 1,is pressed against a rotating polishing pad, 2, with a total forceF_(T), 6. The wafer is held firmly by a retaining ring affixed to agimbaled, rotating wafer carrier (not depicted in FIG. 1). Chemicallyactive abrasive slurry, 5, is directed onto the pad 2 providing thenecessary abrasive mechanism as depicted schematically in FIG. 1. Thepolishing pad, 2, is typically composed of polyurethane, or polyurethaneimpregnated fiber, and is typically attached to a rigid,temperature-controlled platen. Current thinking holds that during CMPthe wafer is supported by hydrodynamic forces, and by direct supportfrom abrasive particles resting on pad asperities at the pad-waferinterface.

Conventional CMP requires an additional support process referred to as“pad conditioning.” Pad conditioning is typically performed withdiamond-impregnated ring or disk tools that are generically referred toas “conditioners” and denoted by 8 in FIG. 1. During conditioning theconditioner, 8, is typically pressed against the rotating polishing pad,2. The pressure and relative motion of the diamond abrasives against thepolishing pad erode a small amount of pad material. Pad erosion isrequired to keep the surface of the pad free of the material build-upassociated with the reaction products of CMP, i.e., spent abrasives, andremoved material from the wafer surface (typically dielectric). Padconditioning also maintains the micro-texture of the pad, which isuseful in that the pad tends to smooth during CMP in response toheat-induced viscoelastic flow of the pad material. Without padconditioning the removal rate and uniformity of the dielectric material(or other material from the surface undergoing planarization) tends tobe variable from wafer-to-wafer making a production-worthy CMP processimpossible. A pad conditioning process whereby pad conditioning andwafer polishing occur simultaneously is referred to as “in-situ”conditioning, as depicted in FIG. 1. When pad conditioning occursfollowing the polishing of one wafer and prior to polishing another, theconditioning process is called “ex-situ” conditioning.

The size of the conditioning tool (or tools) depends on the particularCMP platform being used, but are usually smaller in diameter than thepolishing pad. This situation is depicted in FIG. 1. “Ring-conditioning”tools are usually larger than the wafer diameter. In practice,ring-conditioning tools are positioned at a fixed radial distance fromthe polishing pad's rotational axis and do not undergo oscillatorymotion. At this location the ring-conditioner rotates and provides therequired erosion in the “wafer-track”. The wafer-track is an annularzone on the polishing pad where the oscillating wafer resides duringCMP. “Disk conditioners” are typically smaller than the wafer, their usetypically requires that they oscillate across the pad surface to providethe necessary coverage of the wafer-track.

During pad conditioning, the location and rotation rate of theconditioning tools affect the uniformity of erosion in the wafer-track,and this influences the removal rate stability and polishing uniformityof the dielectric CMP process. In practice, CMP continues for apredetermined time to affect the removal of the desired amount ofmaterial. The appropriate processing time is typically determined bymeans of a calculation making use of knowledge of the material removalrate (for the abrasive, wafer, pad, pressure, etc. in use) and therequired material removal amount. The removal amount is chosen such thatat the end of CMP, the planarized surface is essentially free oftopography (i.e., substantially flat) and has the required finalthickness. Typical industrial CMP processes presently in use havepad-wafer pressures of about 48×10³ Pa (7.0 psi), a relative velocitybetween pad and wafer of about 0.54 meter/sec. and a total processingtime of about 3 minutes. FIG. 2 depicts an enlarged schematiccross-sectional view of CMP in progress on an idealized patterned wafer.

It requires no detailed analysis to appreciate that during CMP regionsof high topography are subjected to a higher localized contact pressurein comparison with low lying regions. Assuming the pad is comprised ofnumerous spring elements, it follows from Hooke's law that compressedpad regions direct larger forces onto the higher regions of the wafertopography. It is this property that gives CMP the ability toselectively remove elevated topographic features (that is, planarize)while minimizing removal in the regions between the raised features.

Current multi-layer IC fabrication schemes typically utilize dielectricCMP processes, including CMP of inter-level dielectric (ILD) materials(typically involving CMP of SiO₂, but may also include organic and/orinorganic low dielectric constant materials), metal CMP processes(typically involving CMP of tungsten, titanium and/or titanium nitride,but may also include various alloys and mixtures of aluminum, copper,tantalum and tungsten). One recent application of CMP referred to as“shallow trench isolation”, or STI involves fabricating isolationstructures around active device areas on the wafer. During STI a blanketdielectric film (typically involving CMP of SiO₂, but may also includeorganic and/or inorganic low dielectric constant materials) is removedso as to reveal inlaid dielectric filled trenches that surround theactive device regions thus providing an electrical isolation “moat”.

Slurries designed for the CMP of conductive layers, such as tungsten andcopper, typically contain Al₂O₃ or SiO₂ abrasive and various chemicaladditives such as acids, oxidizers and buffering agents in an aqueoussuspension to remove the metal layers efficiently by increasing thecapability of chemical polishing. Examples of commercially availabletungsten slurries include Rodel QCCT 1010® and Cabot WA-355®. Slurriesdesigned for oxide layers typically include SiO₂ abrasive in anNH₄OH-based or KOH-based aqueous suspension. Examples of commerciallyavailable SiO₂ slurries include Rodel 1501-50®, Rodel 1508-50®, andCabot SS25®. Commercially available STI slurries from Cabot and Rodelare often based on SiO₂ abrasive, but also may include CeO₂ abrasive.Experimental copper CMP slurries typically contain SiO₂ or Al₂O₃abrasive.

The CMP processes described above may usefully be designated asprecision polishing applications, and thus require a very precise andcontrolled material removal rate across the IC wafer. In addition, thefinal surface quality and smoothness requirements are also ratherdemanding relative to many other polishing processes. In order toachieve these requirements, the abrasive used in the CMP slurry aretypically chosen to have a very precise particle size and shape, and theabrasive must form a very stable suspension so as to avoid particleagglomeration. Deviations from the ideal abrasive specifications oftenresult in poor CMP performance, poor suspension stability and particleagglomeration. Particle agglomeration is particularly troublesomebecause it leads to increased scratching on the surface being polished.Such scratches can lead to electrical shorts across adjacent metal lines(in the case of a scratched ILD layer), defective or damaged conductivestructures (as in metal CMP) or poor electrical isolation (as with STI).

During CMP, dielectric material is typically removed by the combinedaction of chemical and mechanical processes. Chemical energy is suppliedby the slurry's liquid media, or in some cases by the abrasivesthemselves. CMP slurries are typically aqueous based, and are usuallyeither acidic, or basic. Mechanical energy is generated by dragging thedielectric against a semi-rigid surface (the polishing pad) with anabrasive slurry entrained at the wafer-polishing pad interface. Therelative motion generates mechanical energy W in accordance with theprinciple of mechanical work $\begin{matrix}{W = {\int{F_{T}{\mu_{s} \cdot {s}}}}} & (1)\end{matrix}$

Where F_(T) is the total force normal to the wafer surface, μ_(s) is thecoefficient of sliding friction between the wafer and the pad, and ds isa differential element of length. By transforming the differentialelement of length to a differential element of time we obtain$\begin{matrix}{W = {\int{F_{T}{\mu_{s} \cdot v}{t}}}} & (2)\end{matrix}$

where v is the relative linear velocity between the polishing pad andthe wafer. Integration of Eq. (2) yields the total work generated duringthe specified time domain. Thus increasing F_(T), μ_(s), v, or thepolishing time t, increases the material removal during polishing. F. W.Preston recognized the relationship between work and material removaland formalized the relationship in the Preston equation. Stated indifferential form the Preston equation is $\begin{matrix}{\frac{h}{t} = {K_{p}P\frac{s}{t}}} & (3)\end{matrix}$

where K_(p) is the Preston Coefficient, P is the polishing pressureacting normal to the wafer surface, and ds/dt is the instantaneousrelative velocity between the polishing pad and the wafer surface. ThePreston Coefficient is thought to depend on several factors includingthe material type being polished, the concentration and type of abrasiveused in the slurry, and the slurry pH. Equation (3) can be rewritten as

ΔhA=K _(p) F _(T) VΔt  (4)

where A is the total projected contact area. Equation (4) implies thatthe volume of material removed will be proportional to the appliedforce, the relative velocity, and the total process time. The form ofEqs (2) and (4) imply that K_(p) is related to μ_(s). It has beentheorized that removal mechanisms in glass polishing were related to thefrictional force of individual polishing abrasives acting as Hertzianindenters as they traversed the glass surface. In these models themagnitude of these frictional forces was related to the normal loadacting on the individual particles, and to the number of bonding siteson the particle that could interact with the glass surface. The bondingsite density was shown to depend on the pH of the aqueous slurry, andthe iso-electric point (“iep”) of both the glass surface and theabrasive. (O. Kuvaschewsi, E. Evans and C. Alcock, MetallurgicalThermochemistry, Pergamon Press, Oxford (1967); G. A. Parks, Chem. Rev.65, 177, 1965).

Aqueous (water) based slurries have been overwhelmingly preferred in CMPprocesses. In these slurries, the pH is typically adjusted to either theacidic side, or the basic side of the pH scale using appropriatechemicals. Oxidizing agents are typically used in metal CMP slurries toaid in promoting corrosion reactions on metal surfaces. Organic andinorganic acid salts may also be added to improve metal removal rate andremoval of abraded material. Surfactants may also be added to impartstability and to provide lubricity. Chelating agents purportedlyincrease material removal rates from the surface. Co-solvents, such asalcohols or glycols, may have similar, (or the reverse) effects onmaterial removal rates and may be selected as desired by the processengineer for proper material removal. By adjusting the various chemicalaspects of the slurry, the polishing efficiency (removal rates) for agiven abrasive-wafer system can be controlled. For example it has beenshown that solubility of SiO₂ increases as solution pH increases (frompH 7) (R. K. Iler, The Chemistry of Silicon, John Wiley & Sons Inc., NewYork (1979)). Hence high pH slurries are generally more efficientpolishing agents for SiO₂-based materials. This generalization dependssomewhat on the abrasive and the material being polished but illustratesthe impact of chemistry on the slurry performance as predicted by themore sophisticated interpretation of Preston's equation (which may alsoinclude binding site density effects). (F. Preston, J. Soc. Glass Tech.11, 214, 1927).

Currently, CMP abrasives are typically formed solely from onehomogeneous material type. Examples include abrasives formed of Al₂O₃,CeO₂ and SiO₂. These metal oxide abrasives may be segregated into twocategories; chemically active oxides, and chemically inactive oxides.The former pertains to those metal oxide compounds that have multiplevalence states. These compounds can undergo oxidation-reductionreactions under certain circumstances. Cerium (Ce) is an example of ametal with chemically active oxide. For example, ceria (CeO₂) occurs informs in which Ce is in its +3 and +4 valence states, and accordinglycan undergo the following reversible oxidation-reduction reaction:

 2CeO₂⇄Ce₂O₃+O

One approach to the scratching and other problems associated with theuse of abrasive particles described herein makes use of coated abrasiveparticles. The coated particle technology described herein consists of acore particle of one material, and an outer coating of another material.However, for the purposes of this invention the slurry could becomprised of abrasive particles of a homogenous mixture of two or moretypes of materials (in one particle), or the abrasive slurry couldconsist of a mixture of numerous homogenous or composite abrasives.

In addition to the ceria oxidation-reduction noted above, still otherchemical reactions may be possible with other materials, and thus thecoated abrasive technology applies broadly to any number of corematerials and coating materials, and not only metal oxides. It may befeasible to harness such chemical reactions in order to improve theperformance of CMP processes. For example, a slurry comprised ofchemically active abrasive(s) might provide a higher material removalrate for a given amount of mechanical energy input (i.e., polishingpressure), or they may provide a higher removal rate on certain materialtypes while minimizing the material removal rate on another materialtype, and thus provide a desirable removal rate selectivity for somepolishing processes. However, these active materials may be moreexpensive, have a higher material density, or in some cases may haveundesirable impurities relative to SiO₂, or other suitable materials.Thus, their desirable attributes are mitigated to some degree by theirundesirable properties. Hence, finding methods that take advantage ofthe CMP enhancements realized from the chemical properties of certainmaterials while reducing their undesirable attributes would be useful.

The core material can be chosen from any number of organic or inorganicmaterials to reduce cost, density, and to improve the purity of thefinal abrasive particle, whereas the outer coating/shell, or secondaryphase (as with the homogeneous composite particle) may be chosen for itschemical activity/properties.

The particle density will affect its ability to stay in suspensionaccording to Stoke's Equation. Stoke's equation predicts that the heightH that a particle of diameter d will settle in a fluid column in a timet is: $\begin{matrix}{H = \frac{{a^{2}\left( {p_{p} - p_{L}} \right)}{gt}}{18\eta}} & (5)\end{matrix}$

Where η is the fluid viscosity, g is the gravitational constant, andρ_(p) and ρ_(L) are the particle and fluid densities respectively. Thus,for example, a 70 nanometer SiO₂ core (density=2.2), coated with amonolayer ZrO₂ shell (density=5.6), has an effective density of only2.8, and would have a settling rate that is only half that of a solidZrO₂ particle of equivalent size. The density comparison between a solidCeO₂ particle (ρ_(p)=6.95) and a particle with a monolayer of CeO₂coating a 70 nm SiO₂ core is even more dramatic.

Continuing with the ZrO₂ coated SiO₂ core particle described above, thelower density (˜½ that of a solid ZrO₂ particle) will result in nearlytwice the number of slurry particles for a given weight percent (wt %),or solids loading, of abrasive in suspension. Those skilled in the artwill appreciate that, in general, a higher solids loading will increaseremoval rate for a given set of polishing process conditions (polishpressure, polish velocity, slurry chemistry). This is clear from theperspective that more particles present more “cutting” surfaces withwhich to effect material removal. Moreover, by virtue of the largernumber of particles, the penetration depth (scratch depth) of eachindividual abrasive particle is reduced, thus providing a smoother,relatively damage free final surface.

A mathematical model was used to examine the combined effect of wt % ofabrasive, abrasive diameter and polishing pressure on particlepenetration depth (δ), the cumulative contact area between the wafer andall particles in contact with the wafer, and the compressive contactstress (σ) between a single particle and the wafer surface. The resultsare given in FIG. 3. The particle is assumed to have a density of 4.0g/cc and the work surface is taken to be an organic low dielectricconstant material (such as the polyarylene ether, FLARE™ from HoneywellCorporation) that has a modulus of approximately 3 GPa. Results depictedin FIG. 3 indicate that increasing the abrasive solids content of theslurry increases the cumulative contact area between the abrasives andthe wafer surface, while simultaneously reducing the contact stress andpenetration depth (and therefore damage) between the particle and thewafer surface. The composite coated particle having a particular coating(ZrO₂ in this example) will take on the chemical properties (reactivity,isoelectric properties, coordination attributes, etc.) of the solidcoating (ZrO₂). The cost and purity of the abrasive composition willapproach that of the core material present in much higher quantity thanthe coating (SiO₂ in the present example). The mechanical hardnessapproaches that of the surface coating. A useful abrasive is thusconstructed with a hard surface coating surrounding a softer (typicallyless expensive) core material.

The deleterious scratches caused by performing CMP with hard, abrasiveslurries, (and that are sought to be reduced by the present invention)are believed to be chiefly due to the deep penetration of the surface byhard slurry particles. Conventional abrasive particles such as SiO₂,Al₂O₃, CeO₂, SnO₂, or ZrO₂, typically have mechanical strengthscomparable to oxides and tungsten, but tend to be much harder than lowdielectric constant polymers or soft metals such as copper and aluminum.Severe scratches readily result. Commercially available slurries of thetypes noted elsewhere herein are typically made from inorganicconstituents which tend to be reactive to materials such as oxides andmetals but tend not to react with organic materials. Thus, use ofconventional slurries with organic polymers loses much of itseffectiveness in removing materials by chemical means. Thus, abrasiondominates and scratches easily result.

An approach to avoidance of scratches is to make use of soft abrasiveparticles. These soft abrasive particles are selected to have a hardnesscomparable to the hardness of the low dielectric constant materials tobe planarized. Although the use of soft abrasive particles will tend toreduce the role of mechanical abrasion in the CMP process, chemicaleffects may be used to compensate. Soft abrasive particles may be madefrom polymers, including, but not limited to polystyrene-acrylonitrile,Nylon-6, polyoxymethylene, polyurethane and poly(para-divinylphenylene).These soft particles will also typically have a charge opposite that ofthe surface to be polished (opposite zeta potential), increasing therole of chemical polishing and enhancing removal rates. Other kinds oforganic polymer particles, with the appropriate hardness and charge, canbe used to meet the planarization and polishing needs of various lowdielectric constant materials and other soft materials that may bedeveloped.

The abrasive particles may be coated with a thin layer of a softermaterial to reduce scratching where the coating may be chemically activeor inactive material. Typically, these particles' surface charge (zetapotential) will be the opposite of the surface charge of the film to bepolished. The advantage of using coated particles is realized when thedensity of the coated particles is less than the density of particlesmade entirely from the coating material. The reduced density makes theparticles more stable in terms of particle settling according to StokesLaw, which predicts a larger settling velocity for particles having ahigher density. Similarly, for a given wt % of solids, slurriescomprised of coated particles which are less dense will have a greaternumber of particles in a given volume of fluid, referred to as a greatersolids loading. The immediate advantage of a greater solids loading isthat there are more particles to contact the surface of the film to bepolished and therefore the removal rate is increased.

In one embodiment of the present invention, the role of mechanicalabrasion is reduced by using a polishing pad that is of comparablehardness to the low dielectric constant material to be polished, i.e., apolishing pad that is softer than the standard hard IC pad and harderthan the Politex pad. Use of this optimized pad in CMP of low dielectricconstant materials and other soft materials can improve surfacequality—reduce scratches—while achieving acceptable planarity. Theoptimized pad can be developed by adjusting pad production processes,which include baking and curing temperature, time and other parameters.This optimized pad may also be used in a primary polishing step with aconventional, commercially available slurry. Additionally, thisoptimized pad may be used in a primary polishing step with a speciallydesigned slurry, such as the slurries described above, with or withoutconventional or soft abrasive particles.

In another approach to the CMP of the present invention, the primarypolishing step is accomplished with a conventional pad and a slurrycontaining the soft particles described above. Additionally, the softparticles may be used with the chemically reactive slurry componentsdescribed above for the primary polishing step. In another embodiment,the primary polishing step is accomplished with the chemically reactiveslurry described above without the use of soft particles. In anotherembodiment, the chemically reactive slurry described above is used withconventional abrasive particles for the primary polishing step.

In another embodiment, surface scratches that occurred during a primarypolishing step, either a conventional CMP primary polishing step or oneof the primary polishing steps described above, are removed by a buffpolishing step. In one embodiment, a standard, commercially availablesoft pad is used in the buff polishing step. In another embodiment, anoptimized pad as described above is used in the buff polishing step. Inone embodiment, the slurry used in the buff polishing step includes thecomposition described above without abrasive particles. In anotherembodiment the buff polishing step includes the slurry compositiondescribed above with standard or soft abrasive particles. In addition toabrasive slurry particles, reactive chemicals may be contained in theCMP solution. Table A lists typical chemical components that may beincluded in the slurry. The slurry may have the opposite zeta potentialcharge to the surface to be polished, which may typically be comprisedof a low dielectric organic polymer.

TABLE A I. Abrasive Particles 1. Conventional Slurry Abrasive Particles;such as SiO₂, Al₂O₃, CeO₂, SnO₂, ZrO₂. 2. Polymer Particles; such aspolystyrene-acrylonitrile, Nylon-6, polyoxymethylene, polyurethane andpoly(para- divinylphenylene) (“polyPDVP”) 3. Other Particles; chemicallyactive and inactive particles. 4. Particles (1)-(3) above with a softercoating and, preferably, the softer coating has a surface potentialopposite to the film undergoing planarization. II. Solvents 1. Water 2.Organic Solvents; such as anisole, cyclohexanone, N,N-dimethylacetamideN-methyl-2-pyrrolidone, dioxane, tetrahydrofuran, diethyleneglycol. 3.Organic Acids; such as alkyl acids with differing alkyl chain lengthsand structures for various polishing applications including methyl,ethyl, propyl, butyl and cyclohexyl. 4. Organic Bases; such as organicamines having various chain lengths and functional substituentsincluding propyl, butyl, hexyl, octyl, dodectyl, possibly including oneor more hydroxyl groups for increased solubility in water. 5. Otheracids; such as nitric acid, hydrochloric acid, phosphoric acid. III.Other Additives 1. Dispersants; such as lignin sulfonates,sulfosuccinates. 2. Oxidizing Agents; such as ferric nitride, hydrogenperoxide, quinone and substituted quinones. 3. Corrosion Inhibitors;such as benzotriazole (“BTA”), perfluorocarboxylic acid salts,fluorosurfactants. 4. Chelating Agents; such asethylene-diamine-tetra-acedic acid (“EDTA”), citric acid. 5. SurfaceModifying Agents and Wetting Agents; such as phosphate esters, sulfatedethoxylates of fatty alcohols, alkyl phenol polyethoxylates, dimethylsiloxane polymer (with hydrophilic substituent). 6. Surfactants; such aslipophilic/hydrophilic groups linked by alkyl chain having length from 8to 18 carbons including hydroxyl, ammonium and sulfuric,polyoxyethylenealkyl ethers. 7. pH buffers; such as phosphoric acid andcitric acid.

Reactive organic solvents, such as those listed in Table A, can be addedto the slurry to increase the role of chemical polishing in the CMPprocess. Based on the structure and composition of the low dielectricconstant material films, which are often organic polymers, thecorresponding organic solvents are selected to improve the chemicalreaction with the films. The organic solvents of the slurry soften thesurface of the films evenly and break chemical bonds selectively.

Wetting agents, such as those listed in Table A, can be used in theslurry. The organic polymer surfaces typically used in low dielectricconstant films are often hydrophobic—resistant to interaction withwater. Wetting agents can be introduced to improve contact between thelow dielectric constant film and the solvent, which can enhance the roleof chemical polishing.

If particles are used in the slurry, chemical dispersants such as thoselisted in Table A, can be introduced in the slurry to prevent theformation of large particles during the polishing. Without largerparticles in the slurry, smaller particles will not cause severe surfacescratches. This can reduce mechanical abrasion and can reduce surfacescratches.

In summary, the present invention provides several techniques foravoiding or reducing surface scratching in the planarization of softmaterials, typically low dielectric constant materials as encountered inthe fabrication of integrated circuits. On embodiment entails making useof a polishing pad with moderate hardness as a compromise betweensmoothness of surface and effective rates of material removal.Intermediate hardness can be achieved by modification of the process ofpad manufacture, including baking and curing temperatures and times aswell as other pad processing parameters. Such a pad may make use oftraditional slurries or slurries modified as noted above.

Another embodiment makes use of soft slurry particles such as organicpolymer particles, conventional slurry particles coated with a softercoating, in conjunction with other chemically active or inertcomponents. The possible particles and chemical compositions are givenabove, especially in Table A. With such a tailored slurry, planarizationmay be performed with a commercially available, or a tailored pad may beemployed. Slurry particles having an opposite surface from that carriedby the surface undergoing planarization is also advantageous.

Avoidance or reduction of scratching may be achieved by means of aslurry lacking abrasive particles, relying on chemical material removalto achieve planarization. Table A gives possible chemical compositionsfor such planarization, to be performed with a conventional polishingpad or one having controlled, intermediate hardness as described above.

Yet another approach to the reduction/avoidance of scratches of softsurfaces during planarization makes use of a subsequent “buffing” stepfollowing surface planarization. That is, following conventionalplanarization, (or following any of the special soft planarizationtechniques described herein), a relatively mild buffing step isperformed. It is envisioned that the buffing step will remove abrasiveparticles remaining on the surface following the primary planarization,as well as remove scratches. Thus, the buffing step need not be capableof removing the material needed for overall planarization. Rather thebuffing step need only remove enough material for a final planarizationof the surface. Conventional or tailored polishing pads may be used.Soft-slurry or no-slurry (chemical only) compositions may be used inthis buffing step.

Having described the invention in detail, those skilled in the art willappreciate that, given the present disclosure, modifications may be madeto the invention without departing from the spirit of the inventiveconcept described herein. Therefore, it is not intended that the scopeof the invention be limited to the specific and preferred embodimentsillustrated and described. Rather, it is intended that the scope of theinvention be determined by the appended claims.

We claim:
 1. An abrasive slurry for the planarization of a lowdielectric constant surface comprising: a plurality of core particles,wherein each core particle comprises a surface and at least one polymercore material; and a coating material that coats the surface of theplurality of core particles, wherein the core particles and the coatingmaterial form a plurality of abrasive particles and wherein the densityof the coating material is less than the density of the core material.2. The abrasive slurry of claim 1, wherein the plurality of abrasiveparticles has a hardness comparable to the hardness of the lowdielectric constant surface.
 3. The abrasive slurry of claim 2, whereinthe plurality of abrasive particles has a surface charge opposite tothat of the low dielectric constant surface.
 4. The abrasive slurry ofclaim 1, wherein the plurality of abrasive particles has a surfacecharge opposite to that of the low dielectric constant surface.
 5. Theabrasive slurry of claim 1, wherein the core material comprisespolystyrene-acrylonitrile, Nylon-6, polyoxymethylene, polyurethane andpoly(para-divinylphenylene) or mixtures thereof.
 6. The abrasive slurryof claim 1, wherein the coating material is softer than the corematerial.
 7. An abrasive slurry for the planarization of a lowdielectric constant surface comprising: a plurality of core particles,wherein each core particle comprises a surface and at least one corematerial; and a coating material that coats the surface of the pluralityof core particles, wherein the core particles and the coating materialform a plurality of abrasive particles and wherein the density of thecoating material is less than the density of the core material.
 8. Theabrasive slurry of claim 7, wherein the plurality of abrasive particleshas a hardness comparable to the hardness of the low dielectric constantsurface.
 9. The abrasive slurry of claim 7, wherein the plurality ofabrasive particles has a surface charge opposite to that of the lowdielectric constant surface.
 10. The abrasive slurry of claim 9, whereinthe plurality of abrasive particles has a surface charge opposite tothat of the low dielectric constant surface.
 11. The abrasive slurry ofclaim 7, wherein the core material comprises polystyrene-acrylonitrile,Nylon-6, polyoxymethylene, polyurethane and poly(para-divinylphenylene)or mixtures thereof.
 12. The abrasive slurry of claim 7, wherein thecoating material is softer than the core material.